Provenance and Tectonic Setting of the Neogene Clastic Sedimentary Rocks in the Offshore Bay of Bengal, Bangladesh

Integrated tectonostratigraphic analysis of the Himalaya and implications for its tectonic reconstruction

Detrital zircon ages and provenance of Neogene foreland basin sediments of the Karnali River section, Western Nepal Himalaya

The Bengal Basin preserves the erosional signals of coupled tectonic‐climatic change during late Cenozoic development of the Himalayan orogen, yet regional correlation and interpretation of these signals remains incomplete. We present a new geologic map of fluvial‐deltaic deposits of the Indo‐Burman Ranges (IBR), five detrital zircon fission track analyses, and twelve high‐n detrital zircon U‐Pb age distributions (dzUPb) from the Barail (late Eocene–early Miocene), Surma (early–late Miocene), and Tipam (late Miocene–Pliocene) Groups of the ancestral Brahmaputra delta. We use dzUPb statistical tests to correlate the IBR units with equivalent age strata throughout the Bengal Basin. An influx of trans‐Himalayan sediment and the first appearance of ∼50 Ma grains of the Gangdese batholith in the lower Surma Group (∼18–15 Ma) records the early Miocene arrival of the ancestral Brahmaputra delta to the Bengal Basin. Contributions from Himalayan sources systematically decrease up section through the late Miocene as the contribution of Trans‐Himalayan Arc sources increases. The Miocene (∼18–8 Ma) deposition of the Surma Group records upstream expansion of the ancestral Brahmaputra River into southeastern Tibet. Late Miocene (<8 Ma) progradation of the fluvial part of the delta (Tipam Group) routed trans‐Himalayan sediment over the shelf edge to the Nicobar Fan. We propose that Miocene progradation of the ancestral Brahmaputra delta reflects increasing rates of erosion and sea level fall during intensification of the South Asian Monsoon after the Miocene Climate Optimum, contemporaneous with a pulse of tectonic uplift of the Himalayan hinterland and Tibet.

Constraining the collision timing of India and Asia requires reliable information from the coeval geological record along the ~2400 km long collisional margin. This study provides insights into the India–Asia collision at the westernmost margin of the Indian Plate using combined U-Pb geochronological data and sandstone petrography. The study area is situated in the vicinity of Fort Munro, Pakistan, along the western margin of the Indian Plate, and consists of the Paleocene Dunghan Formation and Eocene Ghazij Formation. The U-Pb ages of detrital zircons from the Dunghan Formation are mainly clustered between ~453 and 1100 Ma with a second minor cluster between ~1600 and 2600 Ma. These ages suggest that the major source contributing to the Dunghan Formation was likely derived from basement rocks and the cover sequence exposed mainly in Tethyan Himalaya (TH), Lesser Himalaya (LH), and Higher Himalayan (HH). Petrographic results suggest that the quartz-rich samples from the Dunghan Formation are mineralogically mature and have likely experienced log-distance transportation, which is possible in the case of an already established and well-developed river system delivering the sediments from the Craton Interior provenance. Samples of the overlying Ghazij Formation show a major detrital zircon age clustered at ~272–600 Ma in the lower part of the formation, comparable to the TH. In the middle part, the major cluster is at ~400–1100 Ma, and a minor cluster at ~1600–2600 Ma similar to the age patterns of TH, LH, and HH. However, in the uppermost part of the Ghazij Formation, ages of <100 Ma are recorded along with 110–166 Ma, ~400–1100 Ma, and ~1600–2600 Ma clusters. The <100 Ma ages were mainly attributed to the northern source, which was the Kohistan-Ladakh arc (KLA). The ~110–166 Ma ages are possibly associated with the TH volcanic rocks, ophiolitic source, and Karakoram block (KB). The Paleozoic to Archean-aged zircons in the Ghazij Formation represent an Indian source. This contrasting provenance shift from India to Asia is also reflected in the sandstone petrography, where the sample KZ-09 is plotted in a dissected arc field. By combining the U-Pb ages of the detrital zircons with sandstone petrography, we attribute this provenance change to the Asia–India collision that caused the provenance shift from the southern (Indian Craton) provenance to the northern (KLA and KB) provenance. In view of the upper age limit of the Ghazij Formation, we suggest the onset of Asian–Indian collision along its western part occurred at ca. 50–48 Ma, which is younger than the collision ages reported from central and northwestern segments of the Indian plate margin with 70–59 Ma and 56 Ma, respectively.

ABSTRACTSedimentary deposits of the Cretaceous to Miocene Tansen Group of Lesser Himalayan association in central Nepal record passive-margin sedimentation of the Indian Continent with direct deposition onto eroded Precambrian rocks (Sisne Formation onto Kaligandaki Supergroup rocks), succeeded by the appearance of orogenic detritus as the Indian continent collided with Asia on a N-dipping subduction zone. Rock samples from two field traverses were examined petrographically and through detrital zircon U–Pb dating, one traverse being across the Tansen Group and another across the Higher and Tethyan Himalaya (TH). The Tansen Group depositional ages are well known through fossil assemblages. We examined samples from three units of the Tansen Group (Amile, Bhainskati, and Dumri Formations). The Sedimentary petrographic data and Qt F L and Qm F Lt plots indicate their ‘Quartzose recycled’ nature and classify Tansen sedimentary rocks as ‘recycled orogenic’, suggesting Indian cratonic and Lower Lesser Himalayan (LLH) sediments as the likely source of sediments for the Amile Formation (Am), the TH and the Upper Lesser Himalaya (ULH) as the source for the Bhainskati Formation (Bk), and both the Tethyan and Higher Himalaya (HH) as the major sources for the Dumri Formation (Dm). The Cretaceous–Palaeocene pre-collisional Am is dominated by a broad detrital zircon U–Pb ~1830 Ma age peak with neither Palaeozoic nor Neoproterozoic zircons grains, but hosts a significant proportion (23%) of syndepositional Cretaceous zircons (121–105 Ma) would be contributions from the LLH volcanosedimentary arc, Gangdese batholith (including the Xigaze forearc). The other formations of the Tansen Group are more similar to Tethyan units than to Higher Himalaya Crystalline (HHC). From the analysed samples, there is a lack of distinctive evidence or HH detritus in the Tansen basin. Furthermore, the presence of ~23±1 Ma zircons from the HH unit suggests that they could not have been exposed until the earliest Miocene time.

Documenting basin scale, geometry and provenance through detrital geochemical data: Lessons from the Neoproterozoic to Ordovician Lesser, Greater, and Tethyan Himalayan strata of Bhutan

The Murree Formation is the oldest continental sequence in the Himalayan foreland basin and holds significant information about the Himalayan exhumation following the India-Asia collision. This study explains the provenance of the Murree Formation using detrital zircon U-Pb geochronology supplemented with sandstone petrography to unravel the Himalayan exhumation in Pakistan. The sandstone provenance indicates the sediment was sourced from recycled orogen and suture belts, supporting a post-collisional origin. The source rocks, shown by the ternary plot (QpQnuQu), comprise plutonic and medium to high-rank metamorphic rocks prevalent in the Higher Himalaya (HH), Tethyan Himalayan (TH), and Lesser Himalaya (LH). The zircon ages are mainly ~ 1200–400 Ma, with two minor clusters (~1900–1400 Ma and ~ 2600–2300 Ma). This pattern is compatible with a Himalayan source mainly from the TH, HH, and partly from LH. Notably, the age populations < 200 Ma are in minor percentages, which indicates the hindrance from the Kohistan – Ladakh Arc (KLA) (Asian source) to the foreland basin due to the uplift of the TH and HH blocks. Combining the petrography and zircon U – Pb geochronology, a tectonic model is proposed, which suggests fold – thrust belt propagation to the south after the India – Asia collision. During the deposition of the Murree Formation in the foreland basin, TH and HH blocks were already uplifted in the western Himalaya, which provided major detritus. Along with this significant contribution, a minor contribution from a part of the LH is also suggested. This suggestion is based on the presence of Cambro – Ordovician detrital zircon ages, mainly from the plutonic source indicative of erosion of the Cambro – Ordovician granites and metamorphic rocks due to the initial phase of uplift along the Panjal fault.

This paper deals with the provenance analysis of the Neogene foreland basin sediments of the Siwalik Group in the Nepal Himalaya. This study adopts the techniques of the optical petrography and detrital zircon U–Pb ages from two river sections: the Koshi Nadi in eastern Nepal and the Surai Khola in western Nepal Himalaya. The optical petrography data and resulting QFL plot show a “recycled orogeny” field for the studied sandstone samples, indicating northern lithotectonic units; Tethys Himalaya, Higher Himalaya and Lesser Himalaya as the source of the foreland basin sediments. The detrital zircon geochronological data set has clearly revealed that the cluster ages are younger than ~1000 Ma; however, the older grains (>1000 Ma) are significantly fewer. The obtained age spectrum is similar to the Tethys Himalaya and the upper Lesser Himalaya, but the lower Lesser Himalayan rocks were not distinct, which indicates that sediments in the Neogene foreland basin of the Nepal Himalaya were primarily sourced from the Tethys Himalaya and upper Lesser Himalaya. The minor subordinate scattered peaks that roughly correspond to the age of the Higher Himalaya and lower Lesser Himalaya may indicate that a lower proportion of the sediments might have a link with the Higher Himalaya and lower Lesser Himalaya. Therefore, the provenance of the Siwalik Group in the Nepal Himalaya might have witnessed a mixed type of provenance similar to the northwestern Himalaya.

FIGURE 1. Detrital zircon age spectra from Nyalam High Himalaya (Sample NY93-1) U-Pb age patterns of individual detrital zircons are a potentially powerful tool for constraining metasedimentary provenance. Each zircon grain has a characteristic age reflecting its genesis, and the overall population of detrital zircons represents the spectrum of primary zircon-bearing source rocks, including detritus derived from a number of sedimentary cycles (Cawood et al. 1999). The complex of sillimanite-, kayaniteand garnet-bearing gneiss, calc-silicates and augen gneiss in Nyalam High Himalaya are intruded by granitoids. The garnet-sillimanite paragneisses have experienced an early Paleozoic metamorphism (Gehrel et al. 2003) and Tertiary metamorphism. The source of metasedimentary rocks of the Nyalam High Himalaya zone has been studied using SHRIMP ion microprobe at Beijing SHRIMP Lab, Chinese Academy of Geological Sciences. SHRIMP U/Pb age data from the metasedimentary rocks of the High Himalayan terranes in South Tibet range from ca.23 Ma to 3221 Ma. The U-Pb data allow grouping of the zircons into five major age components with the exception of Cenozoic ages (Figure 1). (1) Archean grains with a maximium age frequency between 3221 and 2509 Ma, (2) Paleoproterozoic grains ranging from 2453 to 1631 Ma, (3) Mesoproterozoic to early Neoproterozoic grains ranging in age between 1530 Ma and 944 Ma, (4) Neoproterozoic grains ranging between 852 and 540 Ma, and (5) Pan-African ages (543-443 Ma). Potential source regions for the detrital zircons occur within the Gondwana terranes: zircons of Archean age correspond to the age of rock unit formed during major a mgmatic and tectonothermal pulses in the Bhandara Craton or Singhbum craton, zircons with paleo-Mesoproterozoic ages reflect tectonic or magmatic events related to or older than the assembly of India (Catlos et al. 2002). Neoproterozoic zircons may have been derived from Lesser Himalayan rocks or the Indian craton (Myrow et al. 2003). Pan-African zircons are the Pan-African orogen event along the Himalayan orogenic belts. The U-Pb age data suggest the sedimentary sources of the studied gneisses in the northern part of the Indian plate. During Late Precambrian and the Palaeozoic, the Gondwanian India bounded to the north by the Cimmerian Super terranes, was part of Gondwana and was separated from Eurasia by the Paleotethys Ocean. During the periods, the northern part of India was affected by a Pan-African event, Numerous granitic intrusions dated at around 500Ma are attributed to this event. The Pan-African event is marked by an unconformity between Ordovician continental conglometrates and the underlying Cambrian marine sediments (Zhu Tongxin et al. 2003). The PanAfrican event form a wide belt stretching from the Alps over the Arabic peninsula, Africa, India, Australia and down to Antarctica. These evidences suggests the presence of the Pan-African orogenic event in the Himalaya. It is tempting to correlate the early Palaeozoic thermal event with a late extensional stage of the long-lasting Pan-African orogenic events, which ended with the formation of the Gondwana supercontinent. The protolith age of the High Himalayan metamorphic rocks is generally regarded to be Precambrian to early Paleozoic. It seems plausible that the High Himalayan metamorphic rocks represent a minimum depositional rage at ∼500 Ma. Our data show strong similarities to previously published spectra for the Greater Himalayan zone, Lesser Himalayan zone and Tethyan Himalayan zone (Parrish et al. 1996; DeCelles et al. 2000; Myrow et al. 2003). Detrital zircon spectra from the High Himalayan range is as young as 500 Ma, so the High Himalaya and Tethyan Himalaya were deposited contemporaneously.

Since the collision between the Indian and Asian plates, several peripheral foreland basin were formed, and started to accumulate the sediments from the hinterland Himalayan orogeny. The sediments deposited at the northern tip of the Greater India have been uplifted, exhumed after the activation of several south propagating thrusts and finally transported to the foreland basin by southward flowing fluvial system. We present petrography and detrital zircon dating for the interpretation of possible provenance of the Neogene Siwalik foreland basin sediments in far western Nepal. The QFL ternary plot for provenance analysis show a 'recycled orogeny' field for the studied sandstone samples, indicating Tethys Himalaya, Higher Himalaya and Lesser Himalaya as the source of the foreland basin sediments. The detrital zircon U-Pb ages of the studied samples have shown that during the time of deposition there was dominant numbers of detritus supplied from the Tethys and upper Lesser Himalaya. Subsequently the amount of the Higher and Lower Lesser Himalaya increased during the time of deposition of the Middle Siwalik.

Tectonic and polymetamorphic history of the Lesser Himalaya in central Nepal

The engineering effect of endogenic processes in Himalayan orogen is a frontier problem of basic scientific research in the field of geosciences and engineering. In the longitude direction, based on the tectonic and lithological differences, the Himalayan orogenic belt is divided into 3 parts: Tethys Himalaya, Higher Himalaya and Lesser Himalaya. These studies reveal the universal law that the engineering geological characteristics and disaster effects of the different parts are restricted by endogenic processes. This paper presents the following arguments. Firstly, the rapid uplift of Higher and Lesser Himalaya under the mechanism of compressional collision and orogeny, the earthquakes are mainly thrust fault type and have high earthquake intensity and high frequency. The dominant direction of geostress is close to NE-WS. The terrain develops to a large height difference. The river cuts strongly. The Mountain disaster is serious. Secondly, the Tethys Himalaya belongs to the detachment system and is in a relatively subsidence state. The earthquakes are mainly normal fault type. The seismicity is relatively weak. The geostress direction is close to the E-S. The terrain evolution is trending towards weakening the terrain. And the avalanche disaster is serious. Lastly, the marine glacial landforms, glacial lakes and glacial debris flows are unique to the Higher Himalaya. These may be important issues in controlling the trans-Himalayan railway line scheme. According to the understanding that there are significant differences in the engineering effects of the above parts, this paper puts forward the suggestion of taking the structural division as the railway engineering geological zoning. Taking the traffic corridor of the proposed China-Nepal railway as an example, the engineering geological zoning map is drawn. The research is helpful to advance the theory of Himalayan orogen to the level of engineering application. At the stage of railway large-scale scheme comparison and selection, it provides a new way to obtain information in wide area, high efficiency and low cost.

Detrital zircon U–Pb age and Hf isotopic composition from foreland sediments of the Assam Basin, NE India: Constraints on sediment provenance and tectonics of the Eastern Himalaya

Previous studies have documented that Sr-Nd and U-Pb zircon isotopic compositions are different between the Lesser Himalayan and the Higher Himalayan metamorphic rocks in Nepal, India, and Pakistan (Parrish and Hodges 1996, Ahmad et al. 2000). However, not much attention was paid to the metamorphic rocks of Main Central Thrust (MCT) zone. We report the result of Sr-Nd isotopic studies in the Higher Himalayan zone, the Lesser Himalayan zone and the MCT zone in far-eastern Nepal. Far-eastern Nepal (Kangchenjunga-Taplejung-Ilam region) comprises three distinct tectonic units: the Higher Himalaya, the Lesser Himalaya and the Sub-Himalaya (Shelling and Arita 1991). Each of these tectonic units is a fault-bound tectonic package. The Higher Himalayan metamorphic rocks consist of mediumto high-grade paragneisses and orthogneisses. The Lesser Himalayan metamorphic rocks comprise primarily metasandstones with intercalations of phyllites and meta-quarzites. The Ilam Nappe is composed of the Higher Himalayan metamorphic rocks. It is a geologically siginificant unit in fareastern Nepal. The Ilam Nappe with no overlying the Tethyan sedimentary rocks has been thrust over the Lesser Himalayan thrust package along the MCT zone up to near the Sub-Himalaya zone in the south. The MCT zone is a ductile-brittle zone with a thickness of less than 1 km to several km. The upper boundary of the MCT zone is known as the Upper MCT (UMCT) and the lower one as the Lower MCT (LMCT). The lithology of the MCT zone is characterized by mylonitic augen gneiss, biotite-muscovitechlorite phyllite with S-shaped garnet and graphitic phyllite. Compositions and zoning patterns of garnets can be discriminated between the MCT zone and the Higher Himalaya. Information on the tectonic disposition prior to the Himalayan orogeny of the geologic units juxtaposed by the MCT is important for understanding the crustal shortening and changes of thermal structure due to the MCT activity. In general, the Higher Himalayan sequence has been considered to be Indian basement in origin, and the Lesser Himalayan sequence has been deposited on the northern margin of the Indian continent in the Precambrian times. However, the Higher Himalayan sequence in the Langtang area, central Nepal yields zircon U-Pb ages of 0.8 to 1.0 Ga, implying a sedimentary provenance of the Late Proterozoic. On the other hand, the Lesser Himalayan sequence contains 1.8 to 2.6 Ga zircons. Therefore, it was proposed that the Higher Himalayan sequence is metasedimentary rocks that were originally deposited north of continental margin than the Lesser Himalayan sequence (Parrish and Hodges 1996). Furthermore, Nd isotope data are useful in distinguishing between Higher Himalayan and Lesser Himalayan zones (Ahmad et al. 2000). These data show that the Nd values for t=1000 Ma in the Higher Himalayan are -10 to -3, whereas these of the Lesser Himalayan are -17 to -7. We have recalculated Nd values at 1000 Ma as this time corresponds to a “Grenville” thermal event in the Himalayas. 17 Samples in the far-eastern Nepal were collected from the Lesser Himalayan, the Higher Himalayan and the MCT zones by the tectonostratigrapic units (Figure 1). The isotopic work presented in this study follows on the structural and petrological studies of Schelling and Arita (1991). In study area the Nd values for t=1000 Ma from rocks of the Higher Himalaya and the Lesser Himalaya are almost within the range of the previous data except for the Tamar Khola Granite. The Nd values for t=1000 Ma obtained are -10 to -2 in the Higher Himalaya and -17 to -15 in the Lesser Himalaya. The Tamar Khola Granite has the Nd value for t=1000 Ma of –11 and the high 147Sm/144Nd. Most samples from the MCT zone have the middle Nd values for t=1000 Ma (-15 to -11) of the samples from Higher Himalaya and Lesser Himalaya (Figure 2a). Similarly, the 87Sr/86Sr values obtained from the MCT zone show the values between those of Higher Himalayan and Lesser Himalayan zones although a few rocks units of Lesser Himalaya have very high 87Sr/86Sr values (Figure 2b). These data may serve to strengthen the opinion that the Higher Himalayan sequence is not Indian basement in origin. Further, these data may suggest that the rocks of the MCT zone fill a gap between the Higher and Lesser Himalayan sequence although Sm-Nd isotopic signiture from them indicate that they had different sediment sources area. Finally, both the Higher and the Lesser Himalayan sequence, including the rocks of the MCT zone between them, may represent thick clastic successions deposited on the north of the thinned continental margin of the Indian basement.

ABSTRACTThe stratigraphic record of Cenozoic uplift and denudation of the Himalayas is distributed across its peripheral foreland basins, as well as in the sediments of the Ganges–Brahmaputra Delta (GBD) and the Bengal–Nicobar Fan (BNF). Recent interrogation of Miocene–Quaternary sediments of the GBD and BNF advance our knowledge of Himalayan sediment dispersal and its relationship to regional tectonics and climate, but these studies are limited to IODP boreholes from the BNF (IODP 354 and 362, 2015-16) and Quaternary sediment cores from the GBD (NSF-PIRE: Life on a tectonically active delta, 2010-18). We examine a complementary yet understudied stratigraphic record of the Miocene–Pliocene ancestral Brahmaputra Delta in outcrops of the Indo-Burman Ranges fold–thrust belt (IBR) of eastern India. We present detailed lithofacies assemblages of Neogene delta plain (Tipam Group) and intertidal to upper-shelf (Surma Group) deposits of the IBR based on two ∼ 500 m stratigraphic sections. New detrital-apatite fission-track (dAFT) and (U-Th)/He (dAHe) dates from the Surma Group in the IBR help to constrain maximum depositional ages (MDA), thermal histories, and sediment accumulation rates. Three fluvial facies (F1–F3) and four shallow marine to intertidal facies (M1–M4) are delineated based on analog depositional environments of the Holocene–modern GBD. Unreset dAFT and dAHe ages constrain MDA to ∼ 9–11 Ma for the Surma Group, which is bracketed by intensification of turbidite deposition on the eastern BNF (∼ 13.5–6.8 Ma). Two dAHe samples yielded younger (∼ 3 Ma) reset ages that we interpret to record cooling from denudation following burial resetting due to a thicker (∼ 2.2–3.2 km) accumulation of sediments near the depocenter. Thermal modeling of the dAFT and dAHe results using QTQt and HeFTy suggest that late Miocene marginal marine sediment accumulation rates may have ranged from ∼ 0.9 to 1.1 mm/yr near the center of the paleodelta. Thermal modeling results imply postdepositional cooling beginning at ∼ 8–6.5 Ma, interpreted to record onset of exhumation associated with the advancing IBR fold belt. The timing of post-burial exhumation of the IBR strata is consistent with previously published constraints for the avulsion of the paleo-Brahmaputra to the west and a westward shift of turbidite deposition on the BNF that started at ∼ 6.8 Ma. Our results contextualize tectonic controls on basin history, creating a pathway for future investigations into autogenic and climatic drivers of behavior of fluvial systems that can be extracted from the stratigraphic record.